Method for activating metal hydride material

ABSTRACT

A method of activating a hydrogen storage alloy. The method includes the step of contacting the hydrogen storage alloy with an aqueous solution of an alkali metal hydroxide where the concentration of the alkali metal hydroxide is at least about 42 weight percent. The method produces a hydrogen storage alloy with increased surface area.

RELATED APPLICATION INFORMATION

The present invention is a continuation-in-part of U.S. patentapplication Ser. No. 09/395,391, filed on Sep. 13, 1999 now U.S. Pat.No. 6,569,567. U.S. patent application Ser. No. 09/395,391 is herebyincorporated by reference herein.

FIELD OF THE INVENTION

The present invention relates to rechargeable hydrogen storageelectrochemical cells. More particularly, the invention relates to amethod of activating hydrogen storage alloy materials and hydrogenstorage alloy electrodes.

BACKGROUND OF THE INVENTION

Rechargeable electrochemical cells using a hydrogen storage alloy as theactive material for the negative electrode are known in the art. Thenegative electrode is capable of the reversible electrochemical storageof hydrogen. The positive electrode typically comprises a nickelhydroxide active material although other active materials, such asmanganese hydroxide, may be used. The negative and positive electrodesare spaced apart in an alkaline electrolyte. A suitable separator (i.e.,a membrane) may also be positioned between the electrodes. As usedherein the terminology “metal hydride material”, “hydrogen storagealloy”, and “hydrogen absorbing alloy” are synonymous.

Upon application of an electrical current to the negative electrode, theactive metal hydride material is charged by the absorption of hydrogen.This is shown by reaction (1).M+H₂O+e⁻→M−H+OH⁻(Charging)  (1)

Upon discharge, the stored hydrogen is released by the metal hydridematerial to provide an electric current. This is shown by reaction (2).M−H+OH⁻→M+H₂O+e⁻(Discharging)  (2)

The reactions at a conventional nickel hydroxide positive electrode asutilized in a nickel-metal hydride electrochemical cell are as follows:Ni(OH)₂+OH⁻→NiOOH+H₂O+e⁻(Charging)  (3)NiOOH+H₂O+e⁻→Ni(OH)₂+OH⁻(Discharging)  (4)

Based on the pioneering principles of Stanford R. Ovshinsky, a family ofextremely efficient electrochemical hydrogen storage materials wereformulated. These are the Ti—V—Zr—Ni type active materials such as thosedisclosed in U.S. Pat. No. 4,551,400 (“the '400 Patent”) the disclosureof which is incorporated herein by reference. These materials reversiblyform hydrides in order to store hydrogen. All the materials used in the'400 Patent utilize a generic Ti—V—Ni composition, where at least Ti, V,and Ni are present with at least one or more of Cr, Zr, and Al.

Other examples of metal hydride alloys are provided in U.S. Pat. No.4,728,586 (“the '586 Patent”) the disclosure of which is incorporatedherein by reference. The '586 Patent describes a specific sub-class ofthese Ti—V—Ni—Zr alloys comprising Ti, V, Zr, Ni, and a fifth component,Cr. The '586 patent, mentions the possibility of additives and modifiersbeyond the Ti, V, Zr, Ni, and Cr components of the alloys, and generallydiscusses specific additives and modifiers, the amounts and interactionsof these modifiers, and the particular benefits that could be expectedfrom them. Still other examples of hydrogen absorbing alloys areprovided in U.S. Pat. No. 5,536,591 (“the '591 Patent”), the disclosureof which is incorporated herein by reference. In particular, the '591Patent provides teaching on the type of surface interface at the metalhydride electrode and the nature of catalytic sites ideal for promotinghigh rate discharge.

In part, due to the research into the negative electrode activematerials, the Ovonic nickel-metal hydride batteries have demonstratedexcellent performance characteristics such as power, capacity, chargingefficiency, rate capability and cycle life. Presently, there is anincreasing use of rechargeable nickel-metal hydride batteries in alltypes of portable tools, appliances, and computer devices. As well,there is a growing use of nickel-metal hydride cells in applicationssuch as electric and hybrid-electric vehicles. Many of the new uses forthe nickel-metal hydride cells require that further improvements be madein the cell's performance.

Many of the performance characteristics of a nickel-metal hydride cellare affected by the surface conditions of the active metal hydridematerial used in the cell's negative electrode. For example, the powerof the cell is affected by both the surface composition and the surfacearea of the metal hydride material. The appropriate modification of thesurface composition and/or the surface area can change the surfacekinetics of the hydride reaction so as to lower the charge transferresistance of the material.

Hydrogen storage alloys are sensitive to the formation of oxides and thealloy surfaces comprise, to a great extent, metal oxides. Thecomposition of these oxides depends on many factors including thecomposition, morphology and method of preparation of the hydrogenstorage alloy. Generally, the type of surface oxides which formnaturally and not by design may be detrimental to the performance of thenegative electrode and cell. Oxides at the surface of the hydrogenabsorbing alloy decreases the alloy's catalytic (charge transfer)capabilities, thereby decreasing both the charging and dischargingefficiency of the electrode and cell.

During cell charging, the decreased surface kinetics of the alloy shiftsthe potential at the surface of the electrode so as to increase theevolution of hydrogen gas via the hydrogen evolution reaction:2H₂O+2e⁻→H₂+2OH⁻  (5)

Atomic hydrogen formed at the surface of the negative electrode caneither recombine with another atomic hydrogen and escape as molecularhydrogen gas or it can react with the hydrogen absorbing alloy in theelectrode to form a hydride. If the surface of the hydrogen absorbingalloy is covered with oxides, hydride formation is inhibited andhydrogen evolution is preferred. Electric current (e.g., electrons)applied to the negative electrode for the purpose of charging theelectrode via charging reaction (1) is instead “wasted” in theproduction of hydrogen gas via reaction (5). This decreases the chargingefficiency of the cell and increases the pressure of hydrogen gas withinthe cell. The decreased surface kinetics also increases the chargetransfer resistance of the material and the electrode so that more poweris wasted due to internal dissipation. It is also believed that thesurface oxides polarize the electrode so as to reduce the rate at whichthe cell discharge process proceeds.

Many of the surface oxides are very dense and impermeable to hydrogentransfer thereby increasing the resistance to hydrogen diffusion duringboth the charging and discharging processes. This has a detrimentaleffect on the rate capability of the electrode.

U.S. Pat. No. 4,716,088, the contents of which is incorporated byreference herein, describes a method of “activating” the hydrogenstorage alloy material by immersing the material into a alkalinesolution. This “alkaline etch treatment” modifies the composition andmorphology of the alloy surface so as to improve the electrochemicalactivity of the alloy and the electrodes formed from the alloy.

The activation process modifies the composition of the oxide layer onthe surface of the alloy. The oxide composition depends upon thecomposition of the underlying hydrogen storage alloy as well as thecorrosivity of the different metals which form the alloy. Certain metalssuch as titanium, zirconium and manganese have a greater affinity foroxidation while other metals such as nickel do not oxidize as readily.Oxide composition may also depend upon the specific process used to makethe alloy since certain processes may promote oxidation more thanothers.

It is believed that immersing the hydrogen storage alloy into thealkaline solution at least partially dissolves certain oxides from thealloy surface. The extent of dissolution depends upon the solubility ofthe specific oxide in the alkaline environment. Certain oxides, such asoxides of manganese, vanadium, aluminum and cobalt are readily solublein an alkaline solution while others, such as those of titanium,zirconium and nickel are less soluble.

The alkaline etch treatment modifies the oxide composition of the alloysurface so as to increase the catalytic activity (charge transfercapabilities) of the material. While not wishing to be bound by theory,it is believed that the activation process increases the concentrationof nickel metal at or near the alloy's surface. Increasing the catalyticactivity of the alloy surface lowers the charge transfer resistance ofthe material and electrode. The lowered resistance results in moreefficient battery discharge since there is less power wasted due tointernal dissipation and more power available for battery output. Thelowered resistance also increases the charging efficiency of the cellsince it shifts the voltage on the surface of the negative electrodeaway from the hydrogen evolution potential.

Activation also provides for a “gradual transitioning” in thecomposition and/or oxidation state of the oxide layer from theelectrolyte/oxide interface to the bulk material. For example, the oxidelayer after activation may have a small concentration of solublecomponents near the electrolyte interface but a composition more closelyresembling the bulk material further away from the interface. This“gradient-type” surface may have an electrical and catalytic naturewhich is more suitable for electrochemical charging and discharging.

The activation process disclosed in the '088 Patent describes analkaline etch treatment wherein the temperature of the alkaline solutionas well as the time in which the hydrogen storage alloy is left incontact with the alkaline solution are both variables that affect theresults of the process. The present invention describes an alkaline etchtreatment of a hydrogen absorbing alloy and an alkaline etch treatmentof a hydrogen absorbing alloy electrode wherein the concentration of thealkaline solution is also a result-effective variable which can bevaried to provide an activated hydrogen storage alloy and an activatedhydrogen storage alloy electrode with increased surface area andimproved electrochemical properties.

SUMMARY OF THE INVENTION

An aspect of the present invention is a method of activating a hydrogenstorage alloy, comprising the step of: contacting the hydrogen storagealloy with an aqueous solution of an alkali metal hydroxide having aconcentration of at least about 40 weight percent. The concentration ofthe alkali metal hydroxide may be at least 42 weight percent.Alternately, the hydrogen storage alloy may be contacted with an aqueousalkaline solution having a hydroxide ion concentration of at least10.558 M.

Another aspect of the invention is a method of activating a hydrogenstorage alloy, said method comprising the step of: contacting saidhydrogen storage alloy with an aqueous solution of an alkali metalhydroxide having a concentration of at least 42 weight percent, saidmethod lacking any subsequent additional chemical treatment with aconjugated unsaturated compound.

Another aspect of the invention a method of activating a hydrogenstorage alloy, said method consisting essentially of the step of:contacting said hydrogen storage alloy with an aqueous alkaline solutionhaving a hydroxide ion concentration of at least 10.558 M, said methodlacking any subsequent additional chemical treatment with a conjugatedunsaturated compound.

Another aspect of the present invention is a method of activating ahydrogen storage alloy electrode for an alkaline electrochemical cell,comprising: contacting the electrode with an aqueous solution of analkali metal hydroxide having a concentration of at least about 40weight percent.

Another aspect of the present invention is a hydrogen storage alloyhaving a surface area of at least 4 square meter per gram achievedwithout electrochemical cycling.

Another aspect of the present invention is a hydrogen storage alloyelectrode, comprising: a hydrogen storage alloy affixed to a conductivesubstrate, the electrode having a surface area of at least 4 squaremeter per gram achieved without electrochemical cycling.

Another aspect of the present invention is a process for making ahydrogen absorbing alloy electrode, comprising the steps of: contactinga hydrogen absorbing alloy with an aqueous solution of an alkali metalhydroxide having a concentration of at least about 40 weight percent;and affixing the hydrogen absorbing alloy onto a conductive substrate.

Another aspect of the present invention is a process for making ahydrogen absorbing alloy electrode, comprising the steps of: affixing ahydrogen absorbing alloy onto a conductive substrate to form anunactivated electrode; and contacting the unactivated electrode with anaqueous solution of an alkali metal hydroxide having a concentration ofat least 40 weight percent. dr

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is the potential-pH equilibrium diagram for the systemzirconium-water at 25° C.;

FIG. 2 shows rate capability curves for electrodes activated at atemperature of 100° C. at 30 weight percent KOH and 60 weight percentKOH; and

FIG. 3 shows rate capability curves for electrodes activated at atemperature of 110° C. at 30 weight percent KOH, 45 weight percent KOHand 60 weight percent KOH.

DETAILED DESCRIPTION OF THE INVENTION

Disclosed herein is a method of activating a hydrogen storage alloy anda method of activating a hydrogen absorbing alloy electrode. Theactivation methods of the present invention are referred to “alkalineetch treatments” whereby a hydrogen absorbing alloy material or ahydrogen absorbing alloy electrode (comprising said material) iscontacted with an alkaline solution. Preferably, the alkaline solutionis a highly concentrated aqueous solution of an alkali metal hydroxide.

First, a method of activating a hydrogen absorbing alloy is described.Generally, the method of activation comprises the step of contacting thehydrogen storage alloy with an alkaline solution. Preferably, thealkaline solution is an aqueous solution of an alkali metal hydroxidewhere the concentration of the alkali metal hydroxide is at least about40 weight percent. More preferably, the alkaline solution is an aqueoussolution of an alkali metal hydroxide where the concentration of thealkali metal hydroxide is at least 42 weight percent. The hydrogenstorage alloy may be “contacted” with the alkaline solution by immersingthe hydrogen storage alloy into a container of the alkaline solution.The hydrogen storage alloy may be in the form of a powder. Alternately,the hydrogen storage alloy may be contacted with an aqueous alkalinesolution having a hydroxide ion concentration of at least 10.558 M.

In one embodiment of the invention, it is preferable that after thehydrogen absorbing alloy is contacted with the alkaline solution, thereare no additional subsequent chemical treatment steps for activating thematerial. Preferably, after the hydrogen absorbing alloy is contactedwith the alkaline material there are no additional subsequent chemicaltreatment steps with a conjugated unsaturated compound.

After the hydrogen absorbing alloy is contacted with the alkalinesolution for a sufficient time (a “sufficient” time is preferably a timesufficient to alter the surface oxides so as to increase the surfacekinetics of the hydrogen absorbing alloy material), the hydrogenabsorbing alloy may be separated from the alkaline solution (forexample, by filtration), washed (for example, with deionized water) anddried. The material may then be affixed to a conductive substrate toform a hydrogen storage alloy electrode. The substrate may be anyconductive support for the hydrogen absorbing alloy material. Examplesof substrates include expanded metal, screen, mesh, foil, foam, andplate. The substrates may be formed from conductive materials such asnickel or a nickel alloy, and copper or a copper alloy. The material maybe affixed to the substrate by compaction, such as by one or morerolling mills. Alternatively, the material may be pasted onto thesubstrate. The electrode may be used as the negative electrode in analkaline electrochemical cell such as a nickel-metal hydrideelectrochemical cell.

Also disclosed herein is a method of activating a hydrogen storage alloyelectrode. A hydrogen storage alloy electrode comprises a hydrogenstorage alloy as the active electrode material. The hydrogen storagealloy electrode may be formed by affixing a hydrogen storage alloypowder onto a conductive substrate. As discussed, the hydrogen storagealloy powder may be affixed to the substrate by methods such ascompaction or pasting. The method of activating a hydrogen storage alloyelectrode comprises the step of contacting the electrode with analkaline solution. Preferably, the alkaline solution is an aqueoussolution of an alkali metal hydroxide where the concentration of thealkali metal hydroxide is at least about 40 weight percent.

The contacting step is preferably done prior to sealing the electrodeinside an electrochemical cell. For example, the electrode may be“contacted” with the alkaline solution by immersing the electrode into acontainer of the alkaline solution. After the electrode is contactedwith the alkaline solution for a sufficient period of time (a“sufficient time is preferably a period of time sufficient to alter thesurface oxides so as to increase the surface kinetics of the hydrogenabsorbing alloy electrode), the electrode is removed from the alkalinesolution. It may then be washed (for example, with deionized water) andthen dried. It may then be used as an electrode for an electrochemicalcell (preferably as a negative electrode for an alkaline electrochemicalcell such as a nickel-metal hydride electrochemical cell).

The contacting step may also be done after the electrode is sealedinside the electrochemical cell. For example, the electrode may first besealed inside an electrochemical cell and then be activated by analkaline solution inside the cell.

The alkaline solution used to activate the hydrogen absorbing alloyand/or the hydrogen absorbing alloy electrode is a “concentrated”alkaline solution which is preferably an aqueous solution of an alkalimetal hydroxide having a concentration which is at least about 40 weightpercent. Preferably, the concentration of the alkali metal hydroxide isbetween about 40 weight percent and about 70 weight percent. Morepreferably, the concentration of the alkali metal hydroxide is betweenabout 50 weight percent and about 70 weight percent. Most preferably,the concentration of the alkali metal hydroxide is between about 55weight percent and about 65 weight percent. It is noted that thealkaline solution preferably has an alkali metal hydroxide concentrationwhich is greater than the concentration which will dissolve in water atroom temperature. Such highly concentrated alkaline solutions are nottypically available commercially as “off-the-shelf” products. Instead,they must be made by dissolving a solid alkali metal hydroxide into acontainer of heated water.

Examples of alkali metal hydroxides which may be used include potassiumhydroxide (KOH), sodium hydroxide (NaOH), and lithium hydroxide (LiOH).Mixtures of potassium hydroxide, sodium hydroxide, and lithium hydroxidemay also be used. Preferably, the alkali metal hydroxide is potassiumhydroxide.

In addition to the concentration of the alkali metal hydroxide, theresults of the activation process are also dependent upon thetemperature of the alkaline solution as well as the time in which thealkaline solution is permitted to contact the hydrogen absorbing alloymaterial. The actual temperature and time conditions used in theactivation process depends upon many factors. Examples of such factorsinclude oxide composition, oxide concentration, the composition of thehydrogen absorbing alloy material being etched, the composition of thehydrogen absorbing alloy electrode being etched, the composition of thealkali metal hydroxide used, and the concentration of the alkali metalhydroxide used in the alkaline solution. Typically, a higherconcentration of the alkali metal hydroxide requires a highertemperature to ensure adequate solubility of the alkali metal hydroxidein the aqueous solution. With concentrations of the alkali metalhydroxide of at least 40 weight percent, the temperature of the alkalinesolution is preferably at least about 60° C., more preferably at leastabout 80° C., and most preferably at least about 100° C. An additionalpreferable range is between 105° C. and 155° C. The time of activationis preferably a time which is sufficient to alter the surface oxides soas to increase the surface kinetics of the hydrogen absorbing alloyand/or the hydrogen absorbing alloy electrode. The time of activationmay be between about one hour and about five hours.

As discussed above, the '088 Patent describes an alkaline etch treatmentprocess wherein the temperature of the alkaline solution as well as thetime period in which the alloy or electrode is left in contact with thealkaline solution are both variables which affect the electrochemicalbehavior of the alloy and/or electrode. The alkaline etch treatments ofthe present invention are distinguishable from what is described in the'088 Patent. The present inventors have discovered that the alkali metalhydroxide concentration of the alkaline solution is also aresult-effective variable which may be modified to remarkably andunexpectedly improve the activation processes. In particular, theinstant inventors have discovered that an alkaline etch treatment usingan alkali metal hydroxide concentration of at least about 40 weightpercent provides for an unexpected increase in the surface area of thehydrogen storage alloy and/or the hydrogen storage electrode beyond thatwhich can be achieved through variations in time and temperature alone.

While not wishing to be bound by theory, it is believed that theincreased surface area of the hydrogen storage alloy is due, at least inpart, to an increase in the solubility of the metal oxides with theincreased pH of the alkaline solution used to perform the alkaline etchtreatment of the present invention. Generally, the solubility of a metaloxide in an alkaline solution increases with the pH of the solution.This may be seen by referring to FIG. 1 which shows the potential-pHequilibrium diagram for the system zirconium-water at 25° C. Thedissolution of zirconium oxide ZrO₂ (zirconia) into zirconate ions HZrO₃⁻ may be expressed by the chemical reaction (6):ZrO₂+OH⁻→HZrO₃ ⁻  (6)

Increasing the concentration of an alkali metal hydroxide, such aspotassium hydroxide, in the alkaline solution increases the OH⁻concentration in the solution, driving reaction (6) to the right andincreasing the dissolution of the oxide. Lines A and B of FIG. 1 are thepH-potential equilibrium lines corresponding to the dissolution of thezirconium oxide (as expressed by reaction (6)). They show that thesolubility of zirconia increases with pH. (For example, increasing theweight percentage of the alkali metal hydroxide from about 30 weightpercent to about 60 weight percent increases the pH of the alkalinesolution by about 0.3 pH units, doubling the solubility of the zirconiumoxide).

The increase in pH of the alkaline solution increases the solubility ofthe zirconium oxide and removes more of the soluble oxide componentsfrom the surface of the alloy, thereby increasing its porosity andsurface area. The increase in pH also removes some of the less solubleoxide components (i.e., oxides such as titanium oxide and chromium oxidethat were negligibly soluble in a 30 weight percent KOH solution),thereby further increasing the porosity and surface area as well ascausing changes in the composition of the oxide layer.

Again, while not wishing to be bound by theory, it is further believedthat the increased surface area of the hydrogen storage alloy is alsodue to increased electrochemical corrosion of the unoxidized metalspecies of the hydrogen storage alloy. For example, the corrosion ofzirconium metal to zirconate ions HZrO₃ ⁻ may be expressed as theelectrochemical oxidation-reduction reaction (7):Zr+OH—+2H₂O→HZrO₃ ⁻+2H₂(gas)  (7)

As seen from reaction (7), the oxidation of zirconium metal to zirconateions HZrO₃ ⁻ is accompanied by the reduction of hydrogen ions tohydrogen gas.

The oxidation-reduction reaction (7) may be written as two separatereactions (7a) and (7b) for the oxidation of zirconium metal and thereduction of hydrogen ions, respectively:Zr+5OH—→HZrO₃ ⁻+2H₂O+4e⁻  (7a)4H₂O+4e⁻→2H₂(gas)+4OH⁻  (7b)

The standard electrode potential E^(o) of the oxidation reaction (7a) ismeasured relative to the potential of the standard hydrogen electrodereaction (7b). Generally, metals which are more reactive than hydrogenare assigned negative values of E^(o) and are said to be “anodic” tohydrogen. Furthermore, the larger the negative potential relative tohydrogen, the more reactive the metal.

With this in mind, the reader is again referred to FIG. 1. Line C is thepH-potential equilibrium line corresponding to the oxidation half-cellreaction (7a) of zirconium metal to zirconate ions HZrO₃ ⁻. Line D isthe pH-potential line corresponding to the reduction half-cell reaction(7b) of hydrogen ions to hydrogen gas.

As may be observed, at sufficiently high values of pH, increases in thepH makes the potential of the oxidation reaction (7a) more negativerelative to the reduction reaction (7b) so that zirconium metal becomesmore reactive relative to hydrogen. Hence, increases in pH increases thecorrosion of the zirconium metal (oxidation reaction 7a) as well as theevolution of hydrogen gas (reduction reaction 7b). Increased evolutionof hydrogen gas increases hydrogen gas pressure. This causes greaterpenetration of the hydrogen gas into the hydrogen storage alloyresulting in cracking and breakage of the alloy particles and increasingthe surface area of the material. The penetration of the hydrogen gasinto the hydrogen storage alloy also causes partial charging of thealloy. (For example, increasing the alkali metal hydroxideconcentration, such as KOH, from about 30 wt % to about 60 wt %increases the pH of the alkaline solution by about 0.3 pH units. Thisincreases the potential difference between the two half-cell reactions(7a) and (7b) by about 30 mV and increases the evolution of hydrogen gassufficiently to increase the hydrogen gas pressure by about a factor often).

It is noted that an alkali metal hydroxide concentration below about 40weight percent does not provide for a sufficient increase in either thedissolution of the metal oxide species nor the corrosion of the metal tosignificantly affect the surface area of either the hydrogen absorbingalloy or hydrogen absorbing alloy electrode. As well, an alkali metalhydroxide concentration above about 70 wt % may be undesirable sincethey may be difficult to dissolve such a high concentration of alkalimetal hydroxide without further increases in temperature. Hence, it ispreferable that the alkali metal hydroxide concentration is below about70 wt %, and more preferably that the alkali metal hydroxideconcentration is below about 65 wt %.

EXAMPLE 1 Alloy Activation/BET Analysis

A sample of a hydrogen storage alloy powder having the compositionZr_(26.6)Ti₉V₅Cr₅Mn₁₆Ni₃₈Sn_(0.4) is subjected to an alkaline etchtreatment by being immersed in a 30 wt % KOH aqueous solution, at about110° C., for a time period of about four and one-half hours. A secondsample of the same alloy powder is immersed in a 45 wt % KOH aqueoussolution, at about 110° C., for about three hours. A third sample of thesame alloy powder is immersed in a 60 wt % KOH aqueous solution, atabout 110° C., for about two hours. The Samples are separated from theKOH solutions, rinsed with deionized water, and dried.

The surface area of the powders are measured using BET analysis. The BETresults are shown in Table 1 for the 30 wt %, 45 wt %, and 60 wt %alkaline etch treatments. It is noted that the surface areas listed inthe table are achieved without any electrochemical cycling.

TABLE 1 POWDER SURFACE AREA (BET MEASUREMENT) ALKALINE ETCH TREATMENTWITHOUT ELECTROCHEMICAL CYCLING % Wt KOH Temp Time BET Surface Area 30%110° C. 4.5 hr 3.2 m²/g 45% 110° C. 3.0 hr 5.9 m²/g 60% 110° C. 2.0 hr6.3 m²/g

EXAMPLE 2 Electrode Activation/BET Analysis

Samples of the same hydrogen storage alloy used in Example 1 arecompacted onto conductive substrates to form electrodes. A firstelectrode is subjected to the alkaline etch treatment at a 30 wt % KOHsolution, at a temperature of about 110° C., for a time period of aboutfour and one-half hours. A second electrode is subjected to the alkalineetch treatment at a 45 wt % KOH solution, at a temperature of about 110°C., and for a time period of about three hours. A third electrode issubjected to the alkaline etch treatment at 60 wt % percent KOHsolution, at a temperature of about 110° C., for a time period of abouttwo hours. No electrochemical charge-discharge cycling is performed onany of the electrodes.

The following Table 2 summarizes the time period, temperature, andpercent weight KOH used to activate the negative electrodes. Also shownis the BET surface area measurement for each of the electrodes.

TABLE 2 ELECTRODE SURFACE AREA (BET MEASUREMENT) ALKALINE ETCH TREATMENTWITHOUT ELECTROCHEMICAL CYCLING Wt % KOH Temp Time BET Surface Area 30%110° C. 4.5 hr 2.1 m²/g 45% 110° C. 3.0 hr 2.6 m²/g 60% 110° C. 2.0 hr6.7 m²/g

The results of Table 1 and Table 2 show that the BET surface area of thehydrogen storage alloy as well as the BET surface area of the hydrogenstorage alloy electrode may be significantly increased without the useof any electrochemical cycling by increasing the alkali metal hydroxideconcentration in the alkaline solution. The surface area results shownin Tables 1 and 2 are especially surprising given that the increases inthe surface area are achieved with reduced activation times. Hence, thealkaline etch treatment of the present invention provides for a moreeffective activation process (i.e., the hydrogen absorbing alloymaterial and hydrogen absorbing alloy electrode have higher surfaceareas) as well as a more efficient activation process (i.e., the processis completed in less time).

Furthermore, the results from Table 1 show that hydrogen storage alloyswith a surface area greater than about 4 square meters per gram may beachieved without any electrochemical cycling by using the alkaline etchtreatment of the present invention. Preferably, hydrogen storage alloyshaving a surface area greater than about 5 square meters per gram may beachieved without any electrochemical cycling. More preferably, hydrogenstorage alloys having a surface area greater than about 6 square metersper gram may be achieved without any electrochemical cycling.

As well, the results from Table 2 show that a hydrogen absorbing alloyelectrode with a surface area greater than with a surface area greaterthan about 4 square meters per gram may be achieved without anyelectrochemical cycling by using the alkaline etch treatment of thepresent invention. Preferably, hydrogen storage alloy electrode having asurface area greater than about 5 square meters per gram may be achievedwithout any electrochemical cycling. More preferably, hydrogen storagealloy electrode having a surface area greater than about 6 square metersper gram may be achieved without any electrochemical cycling.

Electrochemical cycling is used by battery manufacturers to increase thesurface area of the hydrogen storage alloys and hydrogen storage alloyelectrodes. During the electrochemical cycling process the electrode ischarged and discharged for a predetermined number of cycles. Chargingand discharging the electrode forces the absorption and desorption ofhydrogen atoms by the hydrogen storage alloy. This causes expansion andcontraction of the alloy which induces stress and forms cracks withinthe alloy material. The cracking increases the surface area and porosityof the alloy material.

The electrochemical cycling process generally involves a relativelycomplex procedure of cycling the electrochemical cell through a numberof charge/discharge cycles at varying charge/discharge rates for certaintimes. The cycling process puts an additional burden on commercialbattery manufacturers by requiring the manufacturers to purchaseequipment in the form of battery chargers and also requires the cost oflabor and utilities to run the equipment. The alkaline etch treatment ofthe present invention provides a method of substantially increasing thesurface area and performance of hydrogen storage alloys and hydrogenstorage electrodes without the need to perform electrochemical cycling.Hence, higher performance materials, electrodes and batteries may bemanufactured faster and less expensively.

EXAMPLE 3 Electrode Activation/AC Impedance Analysis

The surface area of the alloy and electrode presented in Tables 1 and 2above were measured using BET analysis. The surface area of theelectrode may also be calculated from AC impedance analysis. In general,the AC impedance of an electrode is a nyquist plot showing the realportion of electrode impedance on the horizontal axis and the imaginaryportion of electrode impedance on the vertical axis. The impedances aretypically plotted as a function of a range of frequencies starting at ahigh frequency of about 10 kHz and going to a low frequency of about 20uHz. The double layer capacitance C_(dl) of the electrode, calculatedfrom the AC impedance plot, may be used to determine the surface area ofthe electrode.

A hydrogen absorbing alloy having the compositionZr_(26.6)Ti₉V₅Cr₅Mn₁₆Ni₃₈Sn_(0.4) (the same as Examples 1 and 2) is madeinto a powder and compacted onto an expanded metal substrate to form ahydrogen absorbing alloy negative electrodes. The negative electrodesare activated by immersing the electrodes in hot KOH solutions atvarious KOH concentrations, temperatures and times.

In a first experiment, a set of three electrodes is activated at 100° C.A first electrode is activated at 30 wt % KOH for 4½ hours, a secondelectrode is activated at 45 wt % KOH for 3 hours, and a third electrodeis activated at 60 wt % for 2 hours.

In a second experiment, a set of three electrodes is activated at 110°C. A first electrode is activated at 30 wt % for 4½ hours, 45 wt % for 3hours, and 60 wt % for 2 hours. Each of the electrodes is tested in anegative limited tri-electrode cell with nickel hydroxide positiveelectrodes.

The values of C_(dl) are measured for each of the electrodes and thecorresponding electrode surface areas are calculated. The results areshown in Table 3 for the three electrodes activated at 100° C. and forthe three electrodes activated at 110° C. To calculate the surface areafrom the double layer capacitance C_(dl), a specific capacitance of 25uF/cm² is assumed. It is noted that no electrochemical cycling wasperformed on any of the electrodes. It is further noted that the ACimpedance analysis is performed at 80% state of charge.

TABLE 3 ELECTRODE SURFACE AREA (AC IMPEDANCE ANALYSIS) ALKALINE ETCHTREATMENT WITHOUT ELECTROCHEMICAL CYCLING Wt % KOH Temp Time C_(dl)Surface Area 30% 100° C. 4.5 hr .17 farad/gram 0.7 m²/gram 45% 100° C.3.0 hr .32 farad/gram 1.3 m²/gram 60% 100° C. 2.0 hr 1.0 farad/gram 4.0m²/gram 30% 110° C. 4.5 hr .33 farad/gram 1.3 m²/gram 45% 110° C. 3.0 hr.59 farad/gram 2.4 m²/gram 60% 110° C. 2.0 hr 2.0 farad/gram 8.0 m²/gramThe surface areas calculated using AC impedance analysis results (i.e.,the results shown in Table 3) are consistent with the BET measurementsof Table 2. The results of Table 3 show that the surface area of anactivated hydrogen absorbing alloy electrode increases with the alkalimetal hydroxide concentration of the alkaline solution used to performthe alkaline etch treatments. Moreover, the results show that thesurface area of the electrodes increase even though the time ofactivation is decreased. (It is also noted that the surface areaincreases with temperature when both the time of activation and the KOHconcentration are kept constant).

The increase in surface area of the hydrogen absorbing alloy electrodeprovides for significantly improved electrochemical properties of theelectrode. Certain electrochemical properties are directly dependantupon surface areas.

Rate Capability

The rate capability of the electrode is a measure of the electrodecapacity (mAh/g) as a function of the discharge rate (mA/g). The ratecapability depends upon the diffusion coefficient of the hydrogenspecies through the bulk of the active electrode material as well as the“apparent thickness” of the active material. Increasing the surface areaof the electrode (as hence, of the active electrode material) decreasesthe materials apparent thickness resulting in a improved ratecapability.

FIG. 2 shows the rate capability of the set of electrodes activated at100° C. The results are shown for the alkaline etch at 30 wt % KOH andat 60 wt % KOH. FIG. 3 shows the rate capability for the set ofelectrodes activated at 110° C. The results are shown for the alkalineetch at 30 wt %, 45 wt %, and 60 wt %. The results of both FIGS. 2 and 3show that rate capability improves with increased KOH concentration(from 30 wt % to 45 wt %, and from 45 wt % to 60 wt %).

Limiting Current I_(l)

The limiting current I_(l) is the maximum current obtainable from theelectrode. Like the rate capability, the limiting current I_(l) is afunction of the diffusion coefficient of the active material as well asthe surface area of the material. Hence, increasing the surface area ofthe hydrogen absorbing alloy electrode increasing the limiting currentI_(l). The limiting current was measured for the electrodes of Example 3above at 80% state of charge. Table 4 below shows values of limitingcurrent I_(l) for the hydrogen absorbing alloy electrodes etched using30 wt % KOH (for 4.5 hours), 45 wt % KOH (for 3.0 hours), and 60 wt %KOH (for 2.0 hours). Results are shown for both 100° C. and 110° C.

TABLE 4 LIMITING CURRENT I_(l) Wt % KOH Temp Time I_(l) 30% 100° C. 4.5hr .58 amps/gram 45% 100° C. 3.0 hr not measured 60% 100° C. 2.0 hr .84amps/gram 30% 110° C. 4.5 hr .83 amps/gram 45% 110° C. 3.0 hr 1.7amps/gram 60% 110° C. 2.0 hr 2.3 amps/gramThe results of Table 4 shows that the limiting current increases withincreased KOH concentration (i.e., from 30 wt % to 45 wt % and from 45wt % to 60 wt %).

An embodiment of the activation processes of the present invention is analkaline etch treatment using an alkali metal hydroxide concentrationwithin the range from about 40 weight percent to about 70 weight percentand which also maximizes the limiting current of the electrode.

Charge Transfer Resistance R_(ct)

The charge transfer resistance R_(ct) is directly proportional to thediameter of the high frequency semicircle of the AC impedance plot. Thecharge transfer resistance R_(ct) is measure of the surface kinetics ofthe hydrogen absorbing alloy electrode. The surface kinetics depends onboth the catalytic properties of the electrode active material as wellas the electrode surface area. Table 5 shows the values of the chargetransfer resistance R_(ct) for the electrodes of Example 3.

TABLE 5 CHARGE TRANSFER RESISTANCE R_(ct) Wt % KOH Temp Time R_(ct) 30%100° C. 4.5 hr .51 ohms-gram 45% 100° C. 3.0 hr .15 ohms-gram 60% 100°C. 2.0 hr .15 ohms-gram 30% 110° C. 4.5 hr .17 ohms-gram 45% 110° C. 3.0hr .12 ohms-gram 60% 110° C. 2.0 hr .18 ohms-gramThe results of Table 5 shows that the charge transfer resistancedecreases from 30 wt % KOH to 45 wt % KOH.

An embodiment of the activation processes of the present invention is analkaline etch treatment using an alkali metal hydroxide concentrationwithin the range from about 40 weight percent to about 70 weight percentand which also minimizes the charge transfer resistance of theelectrode. In another embodiment, the concentration may preferably bechosen between about 40 weight percent and about 50 weight percent, morepreferably between about 42 weight percent and about 48 weight percent,most preferably between about 43 weight percent and about 47 weightpercent.

While the present invention has been described with respect to specificembodiments thereof, it will be understood that various changes andmodifications may be made within the scope and spirit of the inventionand it is intended that the invention encompass such changes andmodifications as fall within the scope of the appended claims.

1. A method of activating a hydrogen storage alloy, said methodcomprising the step of: contacting said hydrogen storage alloy with anaqueous solution of an alkali metal hydroxide having a concentration ofat least 47 weight percent.
 2. The method of claim 1, werein said alkalimetal hydroxide is chosen from the group consisting of potassiumhydroxide, sodium hydroxide, lithium hydroxide, and mixtures thereof. 3.The method of claim 1, wherein said contacting step is performed for atime period between about 1 and about 5 hours.
 4. The method of claim 1,wherein said contacting step is performed at a temperature is at leastabout 60° C.
 5. The method of claim 1, wherein said contacting step isperformed at a temperature of at least 80° C.
 6. The method of claim 1,wherein said contacting step is performed at a temperature of at least100° C.
 7. The method of claim 1, wherein the concentration of saidalkali metal hydroxide is between 47 weight percent and 50 weightpercent.
 8. A method of activating a hydrogen storage alloy, said methodcomprising the step of: contacting said hydrogen storage alloy with anaqueous alkaline solution having a hydroxide ion concentration of atleast 12.29 M.
 9. The method of claim 8, wherein said aqueous alkalinesolution comprises an alkali metal hydroxide.
 10. The method of claim 8,wherein said alkali metal hydroxide is chosen from the group consistingof potassium hydroxide, sodium hydroxide, lithium hydroxide, andmixtures thereof.
 11. The method of claim 8, wherein said contactingstep is performed for a time period between about 1 and about 5 hours.12. The method of claim 8, wherein said contacting step is performed ata temperature is at least about 60° C.
 13. The method of claim 8,wherein said contacting step is performed at a temperature is at leastabout 80° C.
 14. The method of claim 8, wherein said contacting step isperformed at a temperature of at least 100° C.